The present invention relates generally to thermal analysis of molten metals and in particular to a device and a process for thermal analysis of molten metals.
Industrially used alloys practically always consist of a base metal which has been alloyed with one or more elements. In liquid state, the alloying additives are in most cases soluble in the base metal. The solidification normally takes place within a solidification range which is typical of the alloying composition. Upon solidification, different solid phases are separated from the molten metal, latent heat being released. By following the temperature and the duration in time of the solidification, it is possible to obtain indirectly a reference for the composition of the alloy and its manner of solidification.
The method has been standardised by the use of test cups or crucibles made of refractory material with an integrated expendable thermocouple. The method, which is called thermal analysis, is widely used for iron and aluminium alloys. The cavity in the test cups industrially used is square or circular in cross-section and the test cups are provided with a centrally placed thermocouple. Typical dimensions are 37xc3x9737 mm and a height of 40 mm. The cups are made of shell-moulding sand and have a wall thickness of about 5 mm. The cavity is completely open upwards where the metal is poured when testing. From a test, a great deal of information can be obtained about the molten metal and its behaviour, for instance, when casting. The crucial point is to provide a high degree of repeatability of the testing. In prior art, the repeatability can vary, among other things, depending on the filling degree of the test cup and variations in heat emission by radiation and convection from the upper surface.
One problem is that the centrally placed thermocouple only registers temperature conditions in the centre of the cup where the molten metal is liquid for a fairly long time.
It is desirable to be able to simultaneously follow the temperature at the surface of the test cup and carry out a more detailed analysis of the test piece by comparing the process in the centre and surface of the test cup.
Test cups with one thermocouple placed in the centre and another at the surface are already known. Thus Swiss patent specification 626 450 discloses a crucible receiving a molten metal, a thermocouple being arranged in the molten metal and another in or at the wall of the crucible. In other known examples, use has been made of cylindrical or cubic test cups, the thermocouple at the surface being placed at a distance of 1-3 mm from the wall. One problem is that a small error when placing the peripheral thermocouple makes the measuring result uncertain.
The object of the present invention is to solve these problems and provide a device and a process for thermal analysis of molten metals providing high repeatability and high resolution. Hence the device and the process have the features stated in claims 1 and 4, respectively.
In the device according to the invention, the spherical cavity has a cylindrical duct which is connected at the top and a cylindrical part which is connected at the bottom.
Since the cavity is spherical, the solidification will take place in a concentric manner, which makes the impulses from the solidification to the thermocouple placed in the centre much clearer than in known cylindrical or cubic constructions. By arranging a cylindrical filling duct, in which the molten metal has a shorter time of solidification than in the spherical cavity, the effect of fluctuations in the heat emission from the upper surface due to emission changes upon radiation will be eliminated. Furthermore, variations due to different degrees of filling will be eliminated since the duct is assumed to be constantly filled after the casting of a test piece.
By placing the lower thermocouple in the transition between the spherical cavity and the lower cylindrical part, the position can vary somewhat without disturbing the repeatability. The purpose of the lower cylindrical part is that the molten metal located in the same should solidify relatively rapidly and before the molten metal in the spherical cavity. Hence thermal conduction occurs in solid phase through the lower part and during the major part of the solidification in the spherical cavity. Therefore, the lower thermocouple can indirectly register the thermal conductivity of the alloy in semi solid to solid phase.
This is useful in particular when testing cast-iron alloys where carbon is precipitated in the form of graphite with high thermal conductivity during the solidification. The graphite can be precipitated in different forms which affect the castability and physical properties of the alloy. If the graphite is precipitated in the form of spheroids, the alloy is called nodular iron. If the graphite is precipitated in the form of agglomerates with thin graphite flakes, the alloy is called grey cast iron or flake graphite cast iron. The thermal conductivity of flake graphite cast iron can be up to 25% higher than if the graphite has been precipitated in the form of spheroids. An intermediate form is the so-called dense graphite iron, which is distinguished by the graphite being precipitated in the form of rounded xe2x80x9cplumpxe2x80x9d bar-like forms. Thus the thermal conductivity can be used to analyse the graphite form.
According to the present invention, an indirect measure of the thermal conductivity can be obtained by measuring the difference in temperature between the thermocouple placed in the centre and the thermocouple placed peripherally in the spherical cavity in the transition between the spherical cavity and the cylindrical part. According to a preferred embodiment of the invention, the difference in temperature is registered when the solidus temperature of the alloy has been reached at the thermocouple placed in the centre.